Edit in JSFiddle 

let reducer = (state=0, action) => {
  switch (action.type) {
    case 'INCREASE': return state+1
    case 'DECREASE': return state-1
    default: return state
  }
}

let store = Redux.createStore(reducer)

let unsubscribe = store.subscribe(() => 
  document.getElementById('state_container').innerHTML = store.getState()
)

window.increase = e => store.dispatch({
	type: 'INCREASE'
})

window.decrease = e => store.dispatch({
	type: 'DECREASE'
})

External resources loaded into this fiddle:

<div id='state_container'>0</div>
<button onclick='increase()'>+</button>
<button onclick='decrease()'>-</button>